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Cw-endor investigation of metal-ligand interactions in solution.

Tucker, Richard James 2005. Cw-endor investigation of metal-ligand interactions in solution. PhD Thesis, Cardiff University.

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Electron Nuclear DOuble Resonance (ENDOR) spectroscopy has been employed in this Thesis to study a range of paramagnetic metal-ligand complex systems. The investigations focussed on the observation of conformational changes, solvatochromic effects and weak diastereomeric interactions of the complexes in frozen solution. Using a combination of angular selective ENDOR spectroscopy and DFT calculations, the structure and conformations of the Vlv=0(acac)2 complex in coordinating and non-coordinating solvents was examined. In the non-coordinating solvent (CD2CI2) the complex was found to adopt the expected square pyramidal structure, where the VOH lg distances and coordinates obtained by DFT and ENDOR were in excellent agreement with each other confirming the expected structure. More importantly, in coordinating solvents (such as pyridine) two different stereoisomers of the resulting Vlv=0(acac)2(C5D< N) adduct, which differed in energy by only 3 kJmol"1, were readily identified and discriminated by ENDOR (PCCP 2002). Subtle changes to metal-ligand structures by solvatochromic effects were also examined using Vlv=0(salen) in both coordinating (DMF) and non-coordinating (CD2CI2) solvents. In the non-coordinating solvent (CD2CI2), the expected square pyramidal symmetry of the V=0 ion above the NNOO plane of the ligand was confirmed both by ENDOR and DFT. However, in the coordinating solvent (DMF), a subtle perturbation from square pyramidal structure was observed suggesting than DMF coordinated trans to the vanadyl oxo-ligand, pulling Vlv=0 back into the ligand plane. This was evidenced through analysis of the VO....H j{j distances determined by ENDOR and confirmed by DFT (Chem. Phys. Lett. 2003). ENDOR was also used to explore the weak enantioselective binding between vanadyl based chiral salen complexes (abbreviated to VO(Jacobsen) ) and chiral propylene epoxide. Differences in epoxide binding by enatiomers of the complex was evidenced by changes to the 'H epoxide derived peaks in the ENDOR spectra. These changes were assigned to the small structural differences between the diastereomeric metal-epoxide adducts. Simulation of the spectra revealed differences in the VO...lHepoxjde distances for the diastereomeric pairs, which was confirmed by DFT. While the epoxide molecule was very weakly coordinated, ENDOR measurements of the racemic complex in racemic epoxide nevertheless indicated the preferential coordination of the /- VO(Jacobsen) to /-epoxide. This demonstrates the unique power of the ENDOR technique to resolve weak chiral interactions for which EPR spectroscopy alone lacks sufficient resolution (./ACS 2004). Finally, the diastereomeric interactions between chiral amines and copper based chiral salen complexes (abbreviated to Cu(Jacobsen) ) was investigated, as a comparison with the previous weak interactions in the VO(Jacobsen)-epoxide case. Diastereomeric discrimination was once again evidenced by ENDOR. The slight differences in the Cu-amine distances, constrained by the chirality of the anchoring site, resulted in subtle difference in the spin densities to the ligand nitrogens, which was detected in the ENDOR experiment.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
ISBN: 9781303224324
Date of First Compliant Deposit: 30 March 2016
Last Modified: 12 Feb 2016 23:13

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